Just my 2 cents.
A 30,000 BTU heater is not going to be enough for an un-heated un-insulated 2 car garage which probably has un-insulated doors as well. I tried a 20K and an 8K kerosene heaters and with both running at opposite ends of the garage I did not notice any difference in warmth. I switched to a much larger unit.
I now use a 150,000 BTU torpedo heater that runs on kerosene. Its portable and has its own 7 gal. fuel tank on the bottom. Its like a portable home fuel furnace. My garage is insulated and has a 12'x12' insulated overhead commercial door. When I need to work in the garage in outside temps that are below freezing I go in the garage and fire up the torpedo for 15 minutes before I start. Once in the garage I crack open the garage door for a fresh air supply and can work in a T-shirt. If you don't like the torpedo type heater you'll probably need 2 of the 30K units.
Just my opinion.